Electron probe having a specific shortfocal length magnetic lens and light microscope



March19, 1968 G A RES I 3,374,349

ELECTRON PROBE HAVING A SPECIFIC SHORT-FOCAL LENGTH MAGNETIC LENS ANDLIGHT MICROSCOPE ori inal Filed Jan. 51, 1961 2 Sheets-Sheet 1 IN VENTOR. VIC T 0/? 6. MAC/P55 PATENT AGENT March 19, 1968 3,374,349 FOCALLENGTH MAGNETIC LENS AND LIGHT MICROSCOPE Original Filed Jan. 31, 1961V. G. MACR ES ELECTRON PROBE HAVING A SPECIFIC SHORT 2 Sheets-Sheet 2INVENTOR. VICTOR G. MACPES PATENT AGENT United States Patent 3,374,349ELECTRON PROBE HAVING A SPECIFIC SHORT- FOCAL LENGTH MAGNETIC LENS ANDLIGHT MICROSCOPE Victor G. Viacres, 3385 Louis Road, Palo Alto, Calif.94303 Continuation of applications Ser. No. 368,448, May 11,

1964., and Ser. No. 86,200, Jan. 31, 1961. This application Nov. 14,1966, Ser. No. 594,275

4 Claims. (Cl. 250-495) ABSTRACT OF THE DISCLOSURE An electron probeincluding a support for a specimen to be probed, an electron beamdirected at said specimen and focused thereon by an adjacent short-focallength lens, and a light microscope for observing the probed specimen.

The present application constitutes a continuation of applicants priorapplications, Ser. No. 86,200, filed Jan. 31, 1961, and Ser. No. 368,448filed May 11, 1964, both applications having now been abandoned.

The present invention relates generally to electron optics, and moreparticularly, to electron probes for purposes of microscopic analysis,micro-machining, topical heat treatment or other application involvingirradiation of a specimen or sample by a beam of high velocityelectrons.

Since 1913 when Moseley determined the irradiation of materials by highvelocity electrons produced X-radiation characteristic of the particularmaterial being irradiated, many varied devices embodying such principlehave beenconstructed for spectrographic analysis. For purposes ofmicroscopic analysis, Hillier proposed the focusing of the electrons toa minute area on the irradiated material, and the utilization of suchmicroprobe in conjunction with more or less conventional X-rayspectrographic equipment, this proposal being the subject of UnitedStates Patent No. 2,418,029. More recently, various microprobes havebeen constructed, primarily for application as a microscopic analytictool, but with the concommitant realization that the electronirradiation of material could also be employed for other purposes suchas micro-matching or topical heat treating of the irradiated materialwith but minor operational variations principally in the electron beamintensity.

In the practicalconstruction of electron microprobes, for microscopicanalysis or other purposes, serious performance limitations haveappeared because of the necessity for certain practical designcompromises. As a particular example, it is necessary in a practicalelectron microprobe not only to focus the electron beam at the desiredpoint on the sample or specimen, but it is also highly desirable toprovide for optical viewing of'the irradiated specimen, and finally, itis necessary to provide for egress of the X-radiation emitted from thespecimen or sample. Thus,-it will be seen that not only an electron beambut light and X-radiation converge at a single microscopic area, and theobvious conflicting space requirements have necessitated certain of thementioned design compromises of existing microprobe equipment. Amongothers, the following limitations have been noted:

(1) a long focal length of the objective electron lens resulting inexcessive spherical aberration of the focused electron beam at theirradiated area of the specimen" (2) Extremely complex reflectingoptical systems that (3) Low take-off angle of the emitted X-radiationso that detection inaccuracies result.

Accordingly, it is a general object of the present invention to providean electron probe capable of excellent electron focusing, enablingsimple optical viewing, producing highly accurate X-ray detection whenused for microscopic analysis, and generally facilitating bothconstruction and operation.

It is a particular feature of the invention to provide an electron probeincluding an electron focusing lens of short focal length and a lensgeometry such that spherical aberration is reduced to a practicalminimum.

A related feature is the particular design of the electron -focusinglens so that although its focal length is short, and the specimen to beradiated is consequently in physical proximity thereto, the specimen ispositioned in a substantially field-free region.

Another related feature is the design of the electron focusing lens sothat a simple, refractive-type light microscope can be accommodatedenabling direct optical viewing of the specimen without, however, anyinterference with the electron beam or its focus.

Another feature of the invention relates to the support of the specimenor sample to be irradiated in a position such that good optical viewingis enabled and a relatively high angle of emergence of X-radiation isenabled to thus provide accurate X-ray detection.

Yet a further feature of the invention is to provide a specimen supportor stage which can support a plurality of specimens for alternativemovement into irradiation position.

A related feature is the provision of a fluorescent specimen on thespecimen stage so that the characteristics of the focused electron beamcan be visually indicated through utilization of the light opticalsystem.

Yet another feature of the invention is the provision of a main housingarranged to accommodate the specimen stage, the light microscope andother units in a manner permitting ease of removal and also arranged tofunction as a mounting jig so that appropriate disposition is obtainedupon reassembly.

These as well as other objects and features of the invention will becomemore apparent from a perusal of the following description of theaccompanying drawing wherein:

FIG. 1 is a perspective view of an electron probe apparatus embodyingthis invention, parts thereof being broken away to illustrate interiordetails of construction,

FIG. 2 is a schematic diagram of the electron probe physicallyillustrated in FIG. 1, and

FIG. 3 is an enlarged fragmentary perspective view of the specimen stageincluded as an element of the BIG. 1 structure.

With reference to the drawings, the electron microprobe is housed withina generally tubular body 10 that is somewhat enlarged at its lower end,as shown in FIG. 1. At the upper end of the tubular body 10, an electrongun is mounted to direct a beam of electrons substantially axiallythrough the tubular body. Such electron gun is of conventional nature,being of the type employed in an ordinary electron microscope, and isthus not illustrated physically in FIG. 1, but is only schematicallyillustrated in FIG. 2. As so illustrated, the electron gun, generallyindicated at 12, includes a filament 14 which is suitably heated so asto emit electrons, a control grid 16, and an appropriately formed anode18 operating at a relatively high positive potential so as to effectacceleration of the emitted electrons into a generally pencil-like beamindicated at E. A suitable control indicated at 20 is provided so thatthe grid bias can be adjusted to provide the required electron sourcecharacteristics.

A toroidal magnetic beam-condensing lens 22 is mounted in axialalignment with and spaced relation to the electron gun 12 so as to focusand condense the beam of electrons E also in a manner normally employedin conventional electron microscopes so that further detailed structuralor operational description is not warranted and such condensing lens isonly illustrated schematically in FIG. 2.

The initially condensed electron beam E is thereafter collimated by anapertured plate 24 interposed in the beam path and preferably composedof brass or other non-magnetic material. The size of the aperture 24a insuch plate 24 determines what portion of the beam E is transmittedtherethrough and what portion is directed against the adjoining surfaceof the plate and thus stopped from further transmission. Preferably, asshown in FIG. 1, four distinct plates 24 are employed, each of whichradiates from a central juncture point or hub so as to form a unitarystructure which has the shape of a cross in transverse section. Themultiple-plate structure is carried at the inner end of a member thatextends through the wall of the tubular body 10, is rotatably supportedthereby, and has a suitable knurled handle 26 at its outer extremityenabling manual rotation of one or another of the plates 24 intobeam-collimating position with its aperture 24a aligned with the axis ofthe tubular body 10. Various size apertures 24a can thus be chosen for aparticular microprobe operation, and if desired, one of the plates 24,rather than being apertured, can have a small amount of fluorescentmaterial placed on its surface and suitable cross-hair line indiciaplaced thereover, as indicated at 24b in FIG. 1. If such plate 24 isthen brought into operative position on the axis of the tubular body 10,a rough indication of the desired alignment of the electron beam E isvisually apparent. So that this surface can be viewed easily by theoperator, a suitable window 28 is mounted in the side of the tubularbody 10.

The collimated electron beam E in its continuing traverse downwardthrough the tubular body next passes through an electrostatic deflectingdevice generally indicated at 30 which includes adjustably-supporteddeflection plates 32 whose potential can be adjusted so as to maintainthe electron beam symmetrically centered on its longitudinal axis, orcan be varied to produce scanning of the beam across a specimen S.

As schematically illustrated in FIG. 2, the electron beam E thereaftercontinues in its traverse, diverging slightly in its advance, until itpasses through a second magnetic objective electron lens, generallyindicated at 34, whose general purpose is to focus the beam intosubstantially a point at the surface of a specimen S supportedtherebeyond.

In accordance with the present invention, this magnetic objective lens34 is formed to provide a short focal length, to permit optical viewingof the irradiated specimen S by a simple refractive-type lightmicroscope, and to permit egress of the emitted X radiation. Like themagnetic condensing lens 22, the magnetic objective lens 34 is ofgenerally toroidal configuration including a coil 35 surrounded by ashield 36. More particularly, the internal surface of the torus asformed by the core shield 36 tapers inwardly in the direction ofelectron motion so as to converge on the electron axis at an angleslightly greater than 30. One pole piece 38 of the lens 34 is formed atthe inner extremity of the shield 36 and the other pole piece 40 of thelens 34 is formed in tapered alignment therewith at the inner extremityof the lower core shield 42 that lies in a plane that is substantiallyperpendicular to the axis of the electron beam. Consequently, as can bereadily visualized by reference to FIG. 2, the magnetic center of thelens 34 is located but a very short distance from its lowermost physicalextremity.

As a result of the disposition of the magnetic center of the objectivelens 34, the mentioned speciment S to be irradiated can be positioned inrelatively close proximity thereto. In turn, the lens 34 can thus bedesigned to have a relatively short focal length. In practice, a focallength of 16 millimeters has been successfully utilized. Since it isknown that spherical aberration of an electron optical system increaseswith an increasing focal length, it will be clear that in the presentstructure, it is reduced to a practical minimum.

The lower pole piece 40 of the magnet is provided with an inward anddownward taper, as indicated at 40a, which serves to restrict thefringing fields of the magnetic lens 34 from the space occupied by thespecimen S. Again, in practice, a restriction of the field to less than10 gauss at the specimen has been obtained. In view of such magneticfield restriction, the specimen S can constitute a ferromagneticmaterial yet will not interfere to any appreciable degree with theproper focusing of the electron beam.

The described taper or internal convergence of the objective lens 34permits the introduction of the tubular lens-supporting body 52 of anadjustable light microscope, generally indicated at 50, in a manner suchthat the light optical axis intersects the electron optical axis at thepoint or minute area (approximately 1 micron diameter) of irradiation ofthe specimen S. As shown best in FIG. 1, the light microscope is mountedas a unit on a circular plate 54 arranged for sealing juncture to thecircular lip at the extremity of a cylindrical protuberance 58projecting angularly from the tubular body 10 of the electron probe.When the plate 54 is secured in position by suitable bolts, conventionalviewing binoculars 60 are available to the operator of the device, andthe lens-supporting body 52 projects angularly inwardly toward theelectron beam axis, the lower extremity of the tubular lens-supportingbody terminating just shy of intersection with the moving electrons. Themicroscope incorporates a light source and illumination arrangement suchthat critical illumination can be realized so as to provide maximumcontrast and thus allow the realization of the inherent resolutioncapabilities of the light optical microscope.

To avoid interference with the proper operation of the magneticobjective lens 34, the tubular lens-supporting body 52 of the lightmicroscope 50 is formed of brass or other non-magnetic material andpreferably a small metallic cylindrical shield 62 is attached to theextremity of the light microscope body so as to isolate the electronbeam and thus preclude the influence of electric charges which mayaccumulate on the non-conductive glass lens of the light microscope 50which accumulation might otherwise produce a distorting effect on theelectron focusing action. Preferably, the axis of the light opticalsystem is approximately 30 relative to the electron beam axis andpreferably, as shown, the specimen S is tilted so that its exposedsurface lies in a plane perpendicular to the light optical axis so as toenhance optical viewing.

Such tilted position of the specimen S also enhances the accuracy ofX-ray detection of associated spectrographic equipment when the electronprobe hereinabove described is utilized for microscopic chemicalanalysis. More particularly, as shown in FIG. 2, even though thespecimen S is located physically quite close to the magnetic objectivelens 34, a relatively large X-radiation emergence angle is obtainable.As shown at X in FIG. 2, X-radiation angles up to 35 or 40 areachievable, and as shown in FIG. 1, a suitable window 64 is positionedin the side of the body .10 to permit passage of the X- radiation to anyassociated spectrographic equipment. It may be mentioned that suchspectrographic equipment forms no part of the present invention, in andof itself, and therefore is not described. However, conventionalspectrographic equipment such as mentioned in the Hillier Patent No.2,418,029 can be employed in association with the present electron probeapparatus when microscopic analysis is to be performed.

The specimen S is supported in the described angular disposition on asuitable stage that is illustrated more clearly in FIG. 3 and whichgenerally is arranged to enable selection of specimens S and movement ofa selected specimen into appropriate irradiation position. As shown,each specimen S can be mounted on a small disk 72. In turn, a pluralityof the small specimen disks 72 can be removably supported for rotationabout their own axes on a plurality of seats peripherally located on alarger disk 74. Suitable electrical and mechanical means are provided toraise and lower this larger disk 74 and also to rotate the same so thatone or another of the specimens S can be brought into irradiationposition. In turn, the rotatable large disk 74 is mounted fortranslatory motion with a table 76 slidably mounted on transverse bars78 within the upstanding ends of a carriage 80 and under the control ofa micrometer 82 that projects through the tubular body of the device soas to enable the desired adjustment during operation. The mentionedcarriage 80 is also mounted on bars or rails 84 for movement in adirection at right angles to the translatory motion of the describedtable 76 and is again under the control of a micrometer 86 that ismanually accessible exterior of the tubular body 10. To facilitatefocusing of the electron beam E on the specimens S, one of the lattercan be in the form of fluorescent material. Thus, for precisionalignment of the electron beam E, the fluorescent specimen S can bebrought into irradiation position and appropriately adjusted physicallyuntil by visual observation through the light microscope 50, the desiredelectron optical focus thereon is obtained.

It 'will, of course, be understood that the tubular body 10 must beevacuated to approximately 10* millimeters of mercury and suitablevacuum equipment is accordingly connected thereto in a conventionalmanner which need not here be described. Also, it will be apparent thatall junctures to the tubular body 10 must be appropriately sealed so asto maintain such vacuum during operation.

Quite obviously, various modifications and/or alterations can be made tothe described structure without depaitming from the spirit of theinvention, Accordingly, the foregoing description of one structure is tothe considered as purely exemplary and not in a limiting sense; theactual scope of the invention is to be indicated by reference to theappended claims. 1

What is claimed is:

1. An electron probe which comprises means for forming an electron beam,means for supporting a specimen to be irradiated at a predeterminedposition on the axis of said electron beam, means including a shortfocal length magnetic electron lens disposed entirely between said beamforming means and the specimen for focusing said beam on the specimen,said electron lens having an internal aperture of convergentconfiguration in the direction of electron motion for passage of thebeam therethrough and a lens center closely adjacent the specimen, alight microscope supported for viewing of the irradiated specimen, saidlight microscope having an optical lens supporting tubular body ofnon-magnetic material projecting partially through the interior of saidconvergent electron lens aperture with its axis intersecting the axis ofsaid electron beam at the surface of the irradiated specimen so that thelight passes through the same lens aperture as the electron beam, andmeans to analyze and detect X-rays emitted from said specimen.

2. An electron probe according ot claim 1 which comprises an electronshield disposed between said electron beam and the terminal portion ofsaid tubular light microscope body.

3. An electron probe according to claim 1 wherein saidspecimen-supporting means supports the specimen with its surface 'to beirradiated in a plane perpendicular to the axis of said lightmicroscope.

4. An electron probe which comprises means for forming an electron beam,means for supporting a specimen to be irradiated at a predeterminedposition on the axis of said electron beam, the surface of the specimenbeing disposed at an acute angle relative to said beam axis, and meansincluding a magnetic electron lens disposed entirely between said beamforming means and the specimen for focusing said beam on the specimen,said electron lens including a coil with a surrounding core shieldterminating in pole pieces having an entirely internal convergentconfiguration in the direction of electron motion, and means to analyzeand detect X-rays emitted from said specimen.

References Cited UNITED STATES PATENTS 2,243,403 5/ 1941 Von Ardenne25049.5 2,273,235 2/1942 Von Ardenne 25049.5 2,301,303 11/1942 Marton25049.5 2,392,243 1/ 1946 Hillier 25049.5 2,849,619 8/1958 Eisfeldt250-49.5 2,862,129 11/1958 Van Dorsten 25049.5 X 2,877,353 3/1959Newberry 25049.5 2,916,621 12/1959 Wittry 25049.5 2,944,172 7/1960 Opitzet al. 25049.5 X 2,987,641 6/1961 Wegmann 25049.5 X 3,049,618 8/1962Thome 25049.5

WILLIAM F. LINDQUIST, Primary Examiner.

RALPH G. NILSON, Examiner.

